The present invention relates generally ultraviolet (UV) blocking coatings and, more particularly, to UV blocking coatings for reflecting UV light while concurrently reducing reflections of visible light from the surface of an optical substrate, such as a window, a windscreen or an eyeglass lens.
Exposure to UV light, such as the UV light present in sunlight, can cause a variety of problems. For example, UV light can cause plastic materials, such as acrylic and polycarbonate materials, to craze as a result of photoinduced chemical crosslinking. Thus, plastic windows which are exposed to UV light will generally develop a network of fine cracks as a result of the photoinduced chemical crosslinking. Moreover, exposure to UV light can lead, at least in part, to the formation of cataracts in the eye and can create cellular damage in plants which, in turn, retards the growth rate of the plants. Further, the exposure of the carpeting and upholstery within buildings or vehicles will generally cause the dyes in the carpeting and upholstery to fade over time. Thus, the interior of a vehicle, such as a train, bus, automobile or the like, will typically fade over time.
In an attempt to reduce the deleterious effects of exposure to UV light, plastic windows and lenses have been designed which include chemical UV absorbers, such as organic dyes. The chemical UV absorbers are added to the plastic material in order to reduce, but not eliminate UV-induced damage. For example, vehicles and buildings, such as greenhouses, can include plastic windows which include chemical UV absorbers in order to reduce the UV-induced damage. In addition, plastic eyeglass lenses can include chemical UV absorbers in order to slow the formation of cataracts.
While plastic windows or lenses which include chemical UV absorbers reduce the resulting UV-induced damage, the overall transmittance of the plastic window or lens is typically reduced slightly in comparison to conventional plastic windows or lenses which do not include chemical UV absorbers. As a result, plastic windows which include chemical UV absorbers will also tend to attenuate light, such as visible light, outside of the UV spectrum. In addition, plastic windows which include chemical UV absorbers also generally have a yellow tint which may be unacceptable for certain applications. Moreover, plastic windows which include chemical UV absorbers typically cost significantly more than conventional plastic windows which do not include chemical UV absorbers.